In portable battery chargers, it may be desirable to have multiple inputs that support available input sources or one input that may be configured in one of different ways to support various applications.
In particular, in multi-input devices, one of the inputs may be configured to limit or otherwise control the total input current drawn from a power source. For example, the power source may be the USB port of a computer which is not permitted to deliver more than a given amount of current. In this case, the battery charger should be able to determine and limit the input current. The output current delivered to the battery would be governed by the input current limit and by the step-down ratio of the inductive switching regulator.
The battery size needs to be sufficiently large to receive all of the available but limited input power. For example, in a 2.5 W USB mode (5V @ 500 mA), the available battery charge current may be as high as (5.5V/2.8V)×500 mA or approximately 1 A. Only batteries capable of being charged at that rate would qualify for this system.
For supporting other input sources, it may be desirable to limit or otherwise control the output current of a switching regulator (i.e. the battery current itself), instead of limiting the input current. For example, such input source as a 5V regulated wall adapter is capable of delivering any amount of current defined by a battery current limit. Input current would be governed only by the step-down ratio of the inductive switching regulator.
In a single-input battery charger, it would be desirable to configure the input pin in accordance with a user request to support any available input source.
Also, it would be desirable to automatically limit voltage developed across the battery. For example, in applications using an input source whose current must be limited, an additional mechanism for limiting voltage developed across the battery would provide the absolute maximum charge current.
Therefore, there is a need for a switching battery charger capable of accurately determining the average input current as well as the average output current of an inductor-based converter, and to use this information for controlling input and/or output current.
Moreover, in accordance with a conventional technique, input or output current may be directly sensed using a current sensing resistor. However, the conventional current sensing resistor is large in physical size. Therefore, it occupies substantial space on a circuit board increasing the cost of a converter and causing significant power loss. Accordingly, it would be desirable to determine the input and/or output current without the use of a current sensing resistor.
Finally, there is a need for circuitry and methodology that would combine controlling input and/or output current as described above with limiting voltage developed across a battery for the purpose of charging it under controlled conditions.